Method for the production of glass substrates for magnetic recording mediums

ABSTRACT

A method for the production of a glass substrate for magnetic recording mediums comprises disk-processing, grinding, polishing and cleaning steps and the method is characterized in that the grinding step is carried out using a cooling liquid and a both side-grinding device for grinding a glass substrate for magnetic recording mediums, in which thin plate-like nonferrous metal-bonded grinding wheels, resin-bonded grinding wheels or vitrified bonded grinding wheels, containing abrasive grains are adhered to the surfaces of the upper and lower surface tables of the device. The method for the production of a glass substrate for magnetic recording mediums permits the easy and efficient production of a glass substrate for magnetic recording mediums, the method does not require the use of any multistage cleaning stage under the application of ultrasonics and any acid-cleaning stage and it can thus eliminate the possibility of any contamination.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for the production of aglass substrate for magnetic recording mediums and more specifically toa method for the production of a glass substrate for magnetic recordingmediums, which comprises disk-processing, grinding, polishing andcleaning steps and which permits the inhibition of any contamination ofthe glass substrate with ions such as Fe ions and/or abrasive grainsduring the grinding step to thus easily and efficiently form a glasssubstrate for magnetic recording mediums whose contamination iswell-controlled.

[0003] 2. Description of the Prior Art

[0004] There has been desired for the reduction of the magnetic head'sflying height of a magnetic disk-recording device in order to improvethe recording density of a magnetic recording disk, as the storagecapacity of the magnetic disk-recording device has increasingly beenhigh. To reduce the flying height of the head, there has been desiredfor the development of a substrate for magnetic recording mediums, whichis excellent in the surface smoothness, has only a small amount ofdeposits on the surface thereof and has a substantially small quantityof surface defects.

[0005] As the conventional substrates for magnetic recording mediumsexcellent in the surface smoothness, there have mainly been used thoseeach prepared by plating an aluminum alloy plate with Ni-P and thenpolishing the plated main surface of the plate in a multiple stepprocess.

[0006] However, the magnetic disk-recording device has recently beenadopted even in the portable personal computer such as notebook-sizedpersonal computers and the magnetic recording medium should be rotatedat a high speed on the order of not less than 10,000 rpm in order toimprove the response speed of the magnetic disk-recording device. Forthis reason, there has been desired for the development of a substratefor magnetic disk-recording mediums having high strength capable ofwithstanding such severe conditions. As such a substrate, which cansatisfy the foregoing requirements, there has been adopted a glasssubstrate.

[0007] Such a mainly adopted glass substrate for magnetic recordingmediums includes, for instance, a chemically strengthened glasssubstrate whose strength is improved by a chemical strengtheningtreatment or a crystallized glass substrate prepared by melting andmolding glass to give a glass substrate, maintaining the glass substrateat a high temperature ranging from 600 to 800° C. over a long period oftime to thus partially separate out crystalline phases in the substrate.

[0008] The chemically strengthened glass substrate is, for instance, oneobtained by melting a glass material and forming the melt into a glasssubstrate for chemically strengthened glass substrates, then subjectingthe glass substrate to grinding and polishing treatments, and immersingit into a molten salt of, for instance, sodium nitrate or potassiumnitrate to form a compression stressed layer on the surface layerthereof and to thus improve the breaking strength thereof. Thecrystallized glass substrate is one, which comprises 40 to 80% ofcrystalline glass phases and 20 to 60% of an amorphous glass phase andwhose strength is improved by the action of the crystalline phase.

[0009] Conventionally, in the method for the production of a glasssubstrate for magnetic recording mediums, which comprisesdisk-processing, grinding, polishing and cleaning steps, the foregoingglass substrate is exposed to a variety of contaminants in theseprocessing steps. In particular, in the step (grinding step) forreducing the thickness of a glass material (blank), the step is carriedout using an abrasive liquid containing abrasive grains such ascarborundum, alumina or zirconia while circulating the abrasive liquidand therefore, some of the abrasive grains remain on the surface of theglass substrate. In addition, a serious problem arises such that theglass surface is contaminated with, for instance, Fe since the surfacetable used for the grinding step is made of cast iron and the abrasiveliquid is circulated during the grinding operations.

[0010] If abrasive grains remain on a glass substrate, scratch marks areformed on the glass substrate in the subsequent polishing step and ifthe scratch marks thus generated are not eliminated during the polishingstep, they remain thereon as surface defects.

[0011] Moreover, if contaminants containing Fe still remain even on themagnetic disk provided thereon with a magnetic recording medium as afinal product, the magnetic disk would suffer from such a seriousproblem that blisters are formed on the contaminated portions under theconditions of accelerated test for inspecting the disk for thereliability in a high temperature and humidity environment.

[0012] To remove these contaminants, the polishing step should becarried out in multiple stages while applying ultrasonics and the glasssubstrate should sufficiently be cleaned by immersing it in an inorganicacid such as hydrochloric acid, sulfuric acid, nitric acid or phosphoricacid or an organic acid such as formic acid, oxalic acid, citric acid,tartaric acid or hydroxyacetic acid as a measure for the prevention ofcontamination with Fe, after the completion of the grinding step. Thisresults in the use of a large-scaled device. In addition, effective acidcleaning requires the use of quite serious conditions such as a hightemperature and a high acid concentration. For this reason, such an acidcleaning step require the use of an expensive device having highresistance to acids and sufficient care should be taken to ensure safetyand sanitation.

SUMMARY OF THE INVENTION

[0013] Accordingly, it is an object of the present invention to providea method for easily and efficiently producing a glass substrate formagnetic recording mediums, which does not require the use of anymultistage cleaning stage under the application of ultrasonics and anyacid-cleaning stage and which can thus eliminate the possibility of anycontamination.

[0014] The inventors of this invention have conducted various studies toachieve the foregoing object, have recognized that it is most effectiveto inhibit the contamination of the glass substrate with ions such as Feions and/or abrasive grains during the grinding step, rather than toimprove the method for removing contaminants, have thus found that theforegoing object can be accomplished by the use of a both side-grindingdevice, in which thin plate-like nonferrous grindstones containingabrasive grains are adhered to the surfaces of the upper and lowersurface tables of the device and a cooling liquid, in place of theconventionally used surface tables of cast iron for grinding operationsand an abrasive liquid containing abrasive grains and have thuscompleted the present invention.

[0015] According to an aspect of the present invention, there isprovided a method for the production of a glass substrate for magneticrecording mediums, which comprises disk-processing, grinding, polishingand cleaning steps, the method being characterized in that the grindingstep is carried out using a both side-grinding device, in which thinplate-like nonferrous metal-bonded grinding wheels, resin-bondedgrinding wheels or vitrified bonded grinding wheels, containing abrasivegrains are adhered to the surfaces of the upper and lower surface tablesof the device and a cooling liquid.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The method for the production of a glass substrate for magneticrecording mediums according to the present invention will hereunder bedescribed in more detail.

[0017] The production method of the present invention makes use of athin plate-like nonferrous metal-bonded grinding wheel, resin-bondedgrinding wheel or vitrified bonded grinding wheel, containing abrasivegrains. In this respect, the abrasive grains are not restricted to anyspecific one, but preferably used are, for instance, those comprisingdiamond grains, cubic boron nitride grains, boron carbide grains,silicon carbide grains, zirconia grains or alumina grains, with theabrasive grains containing diamond being particularly preferred.

[0018] The nonferrous metal-bonded grinding wheel usable in theproduction method according to the present invention may be, forinstance, nickel-base material-bonded, tungsten-base material-bonded,cobalt-base material-bonded and bronze-base material-bonded as well asmixtures thereof-bonded ones. Moreover, the resin-bonded grinding wheelusable herein may be, for instance, phenol resin-bonded and polyimideresin-bonded ones. In addition, the vitrified bonded grinding wheel maybe, for instance, glassy material-bonded and ceramic material-bondedones.

[0019] In the production method according to the present invention, thegrain size of the abrasive grains is preferably on the order of 5 to 30μm and the content of the abrasive grains in the grinding wheelpreferably ranges from 0.5 to 3% by volume. In addition, the grindingwheel has a thin plate-like shape and, for instance, it is practicalthat the grinding wheel has a circular disk-like shape whose thicknessranges from 2 to 7 mm and whose diameter ranges from 5 to 30 mm.

[0020] Particularly preferably used in the production method of thepresent invention are metal-bonded diamond grain-containing grindingwheels disclosed in, for instance, Japanese Examined Patent PublicationNos. Sho 60-21942, Sho 61-33890, Sho 61-33891 and Hei 1-33309.

[0021] In the production method of the present invention, the thinplate-like grinding wheels such as those discussed above are used byadhering them to the surfaces of the upper and lower surface tables of aboth side-grinding device for grinding a glass substrate for magneticrecording mediums. In this connection, the thin plate-like grindingwheel is arranged on and adhered to the surface tables as disclosed in,for instance, Registered Design No. 770,021 and Japanese Examined PatentPublication No. Hei 6-22790 and the grinding step is carried out whilecarrying out dressing.

[0022] In the production method of the present invention, the glasscomponents scraped off from the glass substrate during the grindingoperations are accumulated in the circulated cooling liquid and a smallamount of the grinding wheel components are likewise accumulated thereinto thus contaminate the glass substrate. Accordingly, it is preferred tocontinuously remove these components accumulated in the circulatedcooling liquid in the course of the circulation or to intermittentlyremove the components present in the circulated cooling liquid inpredetermined intervals.

[0023] Therefore, in the production method according to the presentinvention, it is preferred to remove solid contents present in thecooling liquid used in the grinding step and to then recycle it in thestep.

[0024] As means for the removal of the solid contents present in thecooling liquid used in the grinding step, there may be listed, forinstance, separation by filtration through a filtering medium such asdiatomaceous earth, paper or cloth; centrifugation; and separation bysedimentation. The efficiency of the separation by sedimentation isconsiderably low as compared with the centrifugation. The separation byfiltration through paper or cloth as a filtering medium has a lowability of separating fine particles as compared with the separation byfiltration through diatomaceous earth. On the other hand, the separationby filtration through diatomaceous earth has a considerably lowfiltration rate if a large amount of the solid contents should beremoved. Accordingly, it is preferred to remove most of particles havinga large particle size, in advance, through the centrifugation and tothen remove fine particles by filtration through diatomaceous earth inhigh efficiency, while taking into consideration the foregoing facts. Ifthe solid contents present in the cooling liquid used in the grindingstep are removed using a separation system comprising a centrifugalseparator and a diatomaceous earth-filtering device arranged in series,the amount of solid contents present in the cooling liquid thus treatedcan be reduced to a level of not more than 0.2 g/L.

[0025] The glass substrate subjected to the foregoing grindingoperations is, for instance, immersed in an ultrasonic-cleaning bath for2 minutes and then dried while devising a measure to keep the substrateclean. If a glass substrate is subjected to the foregoing grindingoperations, the contamination of the glass substrate can sufficiently beinhibited by only a single-stage ultrasonic-cleaning step. In otherwords, a glass substrate, whose contamination is sufficientlycontrolled, can easily be produced without causing any trouble in thesubsequent steps (polishing step).

[0026] A glass substrate for magnetic recording medium can be producedby subjecting a glass material to the foregoing grinding step, followedby cleaning and then subjecting the glass material to polishing andcleaning steps according to the usual method for the production of sucha glass substrate.

[0027] The present invention will hereunder be described in more detailwith reference to the following Examples and Comparative Examples, butthe present invention is not restricted to these specific Examples.

EXAMPLE 1

[0028] According to the usual procedures for the production of a glasssubstrate for magnetic recording mediums, a lithium silicatecrystallized glass plate (TS-10SX available from K. K. OHARA; comprising70 to 80% of quartz-cristobalite and the balance of amorphous glassphases) was subjected to inner and outer diameter processing to give alarge number of doughnut-like substrates each having an outer diameterof 65 mm, an inner diameter of 20 mm and a thickness of 1.2 mm.

[0029] A plurality of circular disk-like metal-bonded diamondgrain-containing grinding wheels, in which the abrasive grains were#1000 diamond grains, the composition of the metal bond wasNi-15Cu-15Sn-0.5P, whose thickness was 5 mm and whose diameter was 15mm, were produced. 3000 Pieces of the resulting metal-bonded diamondgrain-containing grinding wheels were adhered to each surface of theupper and lower surface tables of a both side-grinding device(16B-5L-III available from SPEEDFAM Co., Ltd.). The upper and lowersurface tables were rotated in the opposite directions, while using theforegoing both side-grinding device and Noritake Cool CG-250MD(available from Noritake Co., Ltd.) as a cooling liquid while supplyingcooling water to thus grind the foregoing doughnut-like substrateshaving a thickness of 1.2 mm. The grinding step was carried out underthe following conditions: a grinding pressure of 150 g/cm²; a rotationalnumber of the upper and lower surface tables of 30 rpm; a rinding timeof 15 minutes and a flow rate of the cooling liquid of 10 L/min. Thisprimary grinding provided doughnut-like substrates each having athickness of 0.85 mm. Incidentally, the cooling liquid was filtered,outside the grinding device, using a centrifugal separator CF-150(available from Toto Separator Co., Ltd.) and a diatomaceousearth-filtering device RRF-20TA (available from Mitaka Industry Co.,Ltd.) and the filtered cooling liquid was recycled.

[0030] A large number of circular disk-like metal-bonded diamondgrain-containing grinding wheels were produced, in which the abrasivegrains were #1500 diamond grains, the metal bond composition wasNi-15Cu-15Sn-0.5P, a thickness was 5 mm and the diameter was 15 mm. 3000Pieces of the resulting metal-bonded diamond grain-containing grindingwheels were adhered to each surface of the upper and lower surfacetables of a both side-grinding device (16B-5L-III available fromSPEEDFAM Co., Ltd.). The upper and lower surface tables were rotated inthe opposite directions, while using the foregoing both side-grindingdevice and Noritake Cool CG-250MD (available from Noritake Co., Ltd.) asa cooling liquid while supplying cooling water to thus grind theforegoing doughnut-like substrates having a thickness of 0.85 mm. Thegrinding step was carried out under the following conditions: a grindingpressure of 100 g/cm²; a rotational number of the upper and lowersurface tables of 30 rpm; a grinding time of 12 minutes and a flow rateof the cooling liquid of 10 L/min. This secondary grinding provideddoughnut-like substrates each having a thickness of 0.67 mm.Incidentally, the cooling liquid was filtered, outside the grindingdevice, using a centrifugal separator CF-150 (available from TotoSeparator Co., Ltd.) and a diatomaceous earth-filtering device RRF-20TA(available from Mitaka Industry Co., Ltd.) and the filtered coolingliquid was recycled.

[0031] The doughnut-like glass substrates having a thickness of 0.67 mmand obtained after the foregoing secondary grinding were cleaned byimmersing in an ultrasonic-cleaning bath for 2 minutes, while applyingultrasonics of 28 KHz, at a flow rate of 2 L/min. Thereafter, thesesubstrates were dried under conditions, which could inhibit anycontamination of the substrates.

[0032] Then the doughnut-like substrates (100 sheets) obtained after theforegoing secondary grinding were fitted to a 16B both side-polishingdevice (available from Hamai Co., Ltd.) to thus polish these glasssubstrates using Mirek 801 (CeO₂-containing abrasive material; averagegrain size D₅ ₀=1.5 μm; available from Mitsui Mining & Smelting Co.,Ltd.) as an abrasive material and MHC15A (foamed urethane; availablefrom Rodel Nitta Co., Ltd.) as an abrasion cloth so that the reducedthickness of the glass substrate reached 15 μm per side (primarypolishing step).

[0033] The glass substrates (100 sheets) obtained after the foregoingprimary polishing step were likewise fitted to a 16B both side polishingmachine available from Hamai Co., Ltd. and the both sides of thesubstrates were then polished using an abrasive liquid containing 0.5%by mass of CEP available from Mitsui Mining and Smelting Co., Ltd. (asolid solution comprising 100 parts by mass of cerium oxide and one partby mass of silicon oxide; average particle size D₅ ₀=0.2 μm) as anabrasive material and MHC 14E (foamed urethane) available from RodelNitta Co., Ltd. as an abrasion cloth, under the conditions of anabrasion pressure of 60 g/cm², a rotational number of 30 rpm and anabrasion time of 20 minutes (second polishing stage). Doughnut-likeglass substrates having a thickness of 0.638 mm were produced as aresult of the secondary polishing step.

[0034] The glass substrates polished above were scrub-cleaned over 3steps (for 3 seconds per step) using a cleaning device for glasssubstrates available from SPEEDFAM Co., Ltd. and a sponge disk availablefrom Kanebo Ltd. Then, these glass substrates were subjected todip-cleaning using SPC 397 (weak alkaline cleaning agent) available fromKyodo Fats & Oils Co., Ltd. as a cleaning agent, followed by rinsing, inorder, in 5 baths under the application of ultrasonics using ultra-purewater. Then the glass substrates were immersed in isopropyl alcohol andthereafter dried in the isopropyl alcohol vapor.

[0035] Regarding the scratch marks formed on the surface of the glasssubstrates during the grinding step and remained thereon, shallow oneswere disappeared through the polishing step, but deep scratch marks wereleft even after the polishing step in the form of recesses (pits).Moreover, these recesses had a tendency that they were connected intodotted lines. The surfaces of twenty glass substrates randomly selectedfrom the foregoing 100 pieces of glass substrates obtained after theforegoing single polishing step were observed using a differentialinterference microscope with a magnification of ×125. However, there wasnot observed any defect in this Example.

[0036] With regard to the fine deposits remaining on the surface of thedried glass substrate, the number of deposits having a diameter of 1 to2 μm was determined using a laser type surface defect-detector RZ-3000(available from Hitachi Electronic Engineering Co., Ltd.). The detectionwas carried out using 20 pieces of glass substrates randomly selectedfrom 100 pieces of glass substrates obtained after the foregoing singlepolishing step. The number of deposits having a diameter ranging from 1to 2 μm was found to be 68, in total, (3.4 per glass substrate on theaverage). In addition, these 68 deposits were also analyzed by SEM-EDSand as a result, it was found that any Fe ion could not be detected.

[0037] The glass substrate produced according to the foregoing methodwas free of any contamination with ions such as Fe ions and the degreeof contamination with the abrasive grains was also found to be extremelylow. Accordingly, the glass substrate was found to be quite useful as aglass substrate for magnetic recording mediums.

Comparative EXAMPLE 1

[0038] According to the usual procedures for the production of a glasssubstrate for magnetic recording mediums, a lithium silicatecrystallized glass plate (TS-10SX available from K. K. OHARA; comprising70 to 80% of quartz-cristobalite and the balance of amorphous glassphases) was subjected to inner and outer diameter processing to give alarge number of doughnut-like substrates each having an outer diameterof 65 mm, an inner diameter of 20 mm and a thickness of 1.2 mm.

[0039] In this Comparative Example, there were used a both side-grindingdevice 16B-5L-III provided with upper and lower surface tables of castiron (available from SPEEDFAM Co., Ltd.) as a grinding machine and aliquid containing 150 g/L of #400 SiC abrasive grains as an abrasiveliquid. The foregoing doughnut-like glass substrates having a thicknessof 1.2 mm were subjected to grinding by oppositely rotating the upperand lower surface tables while supplying the foregoing abrasive liquid.The grinding step was carried out under the following conditions: agrinding pressure of 200 g/cm²; a rotational number of the upper andlower surface tables of 30 rpm; a grinding time of 25 minutes and a flowrate of the abrasive liquid of 5 L/min (primary grinding step). Thisprimary grinding step thus provided doughnut-like glass substrates eachhaving a thickness of 0.85 mm.

[0040] Then there were used a both side-grinding device 16B-5L-IIIprovided with upper and lower surface tables of cast iron (availablefrom SPEEDFAM Co., Ltd.) as a grinding machine and a liquid containing100 g/L of #1500 Al₂ O₃ abrasive grains as an abrasive liquid. Theforegoing doughnut-like glass substrates having a thickness of 0.85 mmwere subjected to grinding by oppositely rotating the upper and lowersurface tables while supplying the foregoing abrasive liquid. Thegrinding step was carried out under the following conditions: a grindingpressure of 100 g/cm²; a rotational number of the upper and lowersurface tables of 30 rpm; a grinding time of 30 minutes and a flow rateof the cooling liquid of 5 L/min (secondary grinding step). Thissecondary grinding step thus provided doughnut-like glass substrateseach having a thickness of 0.67 mm.

[0041] The doughnut-like glass substrates having a thickness of 0.67 mmand obtained after the foregoing secondary grinding were cleaned byimmersing in an ultrasonic-cleaning bath for 2 minutes, while applyingultrasonics of 28 KHz, at a flow rate of 2 L/min. Thereafter, thesesubstrates were dried under conditions, which could inhibit anycontamination of the substrates.

[0042] Then the doughnut-like substrates having a thickness of 0.67 mmand obtained after the foregoing secondary grinding were subjected toprimary polishing, secondary polishing, cleaning and drying according tothe same procedures used in Example 1. The surfaces of 20 pieces ofglass substrates randomly selected from the foregoing 100 pieces ofglass substrates obtained after the foregoing single polishing step wereobserved using a differential interference microscope with amagnification of ×125. As a result, there were observed the generationof defects originated from scratch marks formed and remaining on thesurfaces of the glass substrates during the grinding step, in 8 piecesof glass substrates out of the selected 20 pieces of glass substrates.

[0043] With regard to the fine deposits remaining on the surface of thedried glass substrate, the number of deposits having a diameter rangingfrom 1 to 2 μm was determined by the same procedures used in Example 1.The detection was carried out using 20 pieces of glass substratesrandomly selected from 100 pieces of glass substrates obtained after theforegoing single polishing step. The number of deposits having adiameter ranging from 1 to 2 μm was found to be 104, in total, (5.2 perglass substrate on the average). In addition, these 104 deposits werealso analyzed by SEM-EDS and as a result, it was found that Fe ionscould be detected in 32 deposits out of the 104 deposits.

[0044] The glass substrate produced according to the foregoing methodwas contaminated with both ions such as Fe ions and the abrasive grains.Thus, the resulting glass substrates for magnetic recording mediums arenot necessarily useful as glass substrates for magnetic recordingmediums.

[0045] As has been described above in detail, the method for theproduction of a glass substrate for magnetic recording mediums accordingto the present invention permits the easy and efficient production of aglass substrate for magnetic recording mediums, the method does notrequire the use of any multistage cleaning stage under the applicationof ultrasonics and any acid-cleaning stage and it can thus eliminate thepossibility of any contamination.

What is claimed is:
 1. A method for the production of a glass substratefor magnetic recording mediums, which comprises disk-processing,grinding, polishing and cleaning steps, the method being characterizedin that the grinding step is carried out using a cooling liquid and aboth side-grinding device for grinding a glass substrate for magneticrecording mediums, in which thin plate-like nonferrous metal-bondedgrinding wheels, resin-bonded grinding wheels or vitrified bondedgrinding wheels, containing abrasive grains are adhered to the surfacesof the upper and lower surface tables of the device.
 2. The method forthe production of a glass substrate for magnetic recording mediums asset forth in claim 1 , wherein the abrasive grains are diamond, cubicboron nitride, boron carbide, silicon carbide, zirconia or aluminagrains.
 3. The method for the production of a glass substrate formagnetic recording mediums as set forth in claim 2 , wherein theabrasive grains are diamond grains.
 4. The method for the production ofa glass substrate for magnetic recording mediums as set forth in claim 1, wherein solid contents present in the cooling liquid used in thegrinding step are removed and then recycled to the device.
 5. The methodfor the production of a glass substrate for magnetic recording mediumsas set forth in claim 2 , wherein solid contents present in the coolingliquid used in the grinding step are removed and then recycled to thedevice.
 6. The method for the production of a glass substrate formagnetic recording mediums as set forth in claim 3 , wherein solidcontents present in the cooling liquid used in the grinding step areremoved and then recycled to the device.
 7. The method for theproduction of a glass substrate for magnetic recording mediums as setforth in claim 4 , wherein the solid contents present in the coolingliquid are removed by filtration through diatomaceous earth as afiltering medium.
 8. The method for the production of a glass substratefor magnetic recording mediums as set forth in claim 5 , wherein thesolid contents present in the cooling liquid are removed by filtrationthrough diatomaceous earth as a filtering medium.
 9. The method for theproduction of a glass substrate for magnetic recording mediums as setforth in claim 6 , wherein the solid contents present in the coolingliquid are removed by filtration through diatomaceous earth as afiltering medium.